7.4v Receiver pack/batteries/servos

Having never used anything other than 4.8v receiver packs, I'm a little unsure as to matching a 2s Lipo to the correct receivers and servos.

I'm looking at an overlander 2200mAh 2S, married to a Futaba R3006SB receiver. I really need some guidance as to which Futaba servos' will match the above, bearing in mind it's going in a PSS model, with 4 servos, and two of them being utilised on tailerons.

The model you describe seems pretty simple to the casual observer and i cant see any advantage in going the lipo route. Is there a reason for ditching the 4.8v nimh?

I just thought that I need a lot of weight in the nose (Mig 29), so would use a lipo rather than lead. Also, I was thinking that the two servos for the tailerons could use a fair bit of power. So would hoping this would be 'useful' weight rather than ballast. If that makes sense.

Have you considered a 2S LiFe pack - you can leave and charge them in the model and although they briefly peak at 7.2V they deliver close to 6.6V for most of their discharge. If your servos are voltage sensitive, a silicon diode of a suitable current rating in series will knock off around 0.6V...intrinsically more reliable than a uBEC.

Nice thing with LIFE is that it charges so quickly compared to NIMH. As said can be left in model too. 2s LIFE is fine on Futaba standard servos and receivers too . Only real downside is balance charging needed so slightly more complex and do need monitoring . I have found LIFE very reliable after using for a couple of seasons. Only problem I had was when one battery went out of balance and wouldn’t charge. This was solved by discharge and then recharging . The battery was a year old though and I’m carefully monitoring it.

................... The advantage of lipos over LiFe is you can test the capacity (voltage versus capacity left). Disadvantage they need to be removed for charging.........................................

Edited By Robert Welford on 22/08/2018 10:33:11

I woud say that removing Lipos for charging is the users choice not a "need". I have been using small 2s Lipos for Rx in models for several years and don't remove them for charging. Balance charge every time and no problems so far.

cant beat a good smirk. And its not just you, the whole argument confuses me, and as you say you an dig a much bigger hole with lead.

Anyway if you use mini servos, digital or otherwise, i suspect they will be quite happy on a 1200 4.8v pack. If you use spektrum then 6v is a better bet

What's confusing about avoiding the use of lead in a model to keep the weight down?

The less mass a model has, the less lift it will need to fly, which means it can get sufficient lift at a lower speed. i.e. lighter models fly better.

If possible, I always adjust the CoG of a model by choosing / using the most appropriate equipment / positioning. I don't think I own a single model with added lead. I dare say I might need to add lead if I flew scale models but I'd always try to use "useful weight" (larger engine maybe) before adding lead.

Gary - War birds are known for needing extra weight up front, and PSS models even more so, no matter how much weight you try to cut off the tail end, or how far forward you mount the RC gear. The PSS guys are currently in the process of a mass build Hawker Hurricane and all of these very experienced builders are having to add extra weight in the nose. Remember, no motor in these babies, just purely wind power.

I am now changing over to 2S LiFe wherever possible but am wary of whether some servos are compatible so if in doubt use a suitable UBEC. They are very much lighter than LiPo`s or NiMh`s so to get the same weight you can use very high capacity ones. A HK Failover switch is an alternative and means that you can use two packs for redundancy. Works well for me. They are almost indestructible unless you forget to disconnect from the Failover and leave for a couple of months!

I still maintain that it's better to control CoG wherever possible by moving equipment around than by adding lead wherever possible. The OP's idea of using a different type of battery up front rather than lead seams perfectly valid to me - as long as it is right up front and not just adding weight to the centre of the model.

I still maintain that it's better to control CoG wherever possible by moving equipment around than by adding lead wherever possible. The OP's idea of using a different type of battery up front rather than lead seams perfectly valid to me - as long as it is right up front and not just adding weight to the centre of the model.

To continue to state the (fairly) obvious, though, for PSS models in particular, it ought to be generally possible to get lead weight very far forward indeed - right into the nose cone. This is probably not possible to such an extent with 'useful' weight like batteries, which have fixed sizes and so will only go so far forward...

Yes I get that lead is inevitable when you have no motor up front, but it's still true that the less lead you add to get the model balanced, the better it will fly (unless you are deliberately adding ballast for flying in high winds etc).

I once bought a power glider which came with two spinners - one for using with the motor and one for stuffing with lead if used as a glider. You can't get the weight much further forward than that.

What's confusing about avoiding the use of lead in a model to keep the weight down?

The less mass a model has, the less lift it will need to fly, which means it can get sufficient lift at a lower speed. i.e. lighter models fly better.

If possible, I always adjust the CoG of a model by choosing / using the most appropriate equipment / positioning. I don't think I own a single model with added lead. I dare say I might need to add lead if I flew scale models but I'd always try to use "useful weight" (larger engine maybe) before adding lead.

up, cant disagree with anything you say...but, if you have an engine that is of adequate power for the model, and is X weight + Y lead to balance, and swap it for an engine that is equal in weight to X+Y the model remains the same weight and yet the engine is likely to have cost you more adding to the overall cost of the model. Its also likely to use more fuel so you need a bigger tank which makes it heavier if anything. The added power is of no use to you as you had enough before. It may also be louder which could upset the man with the noise meter. As an example, this would be like taking a 63 inch spitfire and swapping its perfectly adequate 90fs for a 120fs. In this instance the 120 is likely to make the model more difficult to fly due to its higher static thrust (for landing) and torque swing on takeoff.

I agree that all efforts should be made to minimise ballast by shoving the equipment around and making things lighter down the back end but their comes a point where you just need to add the lead. My Sea Fury is a case in point. 60cc engine in the nose with a heavy ali spinner, as well as two sub-c batteries as far forward as they can go. This was not enough so there is 1.5lbs of lead on board just under the engine. I couldnt have done any better.

I was just making the point that adding lead isn't generally the best way of balancing a model.

Regarding engine choice, I'm one of the flyers who think that a bit of extra power can get you out of trouble if used correctly. I'm also not convinced that a larger engine running at the same power output as a smaller engine will use more fuel or be noisier, as it will be running at much lower revs. Personal preference really, but I'd rather add power than lead.